Japanese Patent Application Publication No. 63-59783 (JP63-59783A) (hereinafter referred to as Patent Document 1) discloses a method of performing drive control of a synchronous electric motor. The Patent Document 1 shows: before the synchronous motor is regularly driven, two-phase DC excitation is applied to the synchronous motor, thereby bringing a magnetic rotor into a standstill at a stable point; then, based on a count value of an incremental encoder at the stable point and a count value of the incremental encoder when the DC excitation is started, an encoder count value corresponding to an original point for a one-phase excitation is calculated; then, an electrical angle phase of the synchronous motor is determined from a difference between the encoder count value and the original point, and using the electrical angle phase, drive control of the synchronous motor is performed. In this conventional art, when the synchronous motor is subjected to an initial operation, it is necessary to bring the rotor into a standstill once at a mechanically stable point, and then to drive the synchronous motor. However, when the rotor is not located at the mechanically stable point, it is necessary to greatly move the rotor to the mechanically stable point. In such cases, the rotor has to be moved greatly, which is a problem. Then, in a control system for a synchronous electric motor disclosed in Japanese Patent Application Publication No. 02-241388 (JP02-241388A) (hereinafter referred to as Patent Document 2), an initial operation of a synchronous motor (determination of a phase angle at which the maximum torque may be obtained) is performed without rotating a rotor. Specifically, in the control system for a synchronous electric motor disclosed in Patent Document 2, torque is detected while changing the phase angle between a permanent magnet pole of the rotor and a rotational magnetic field formed by a stator. Then, a power factor when torque is zero is obtained. Further, a location shifted from the location where the torque is zero just by an electrical angle phase of 90° is defined as a reference point for a commutation angle command with the power factor of one, and an initial operation is performed using the commutation angle command.
In the control system for a synchronous electric motor disclosed in Patent Document 2, a rotor angle in a moving direction of the rotor, at which output torque of the motor finally becomes zero, is sequentially obtained. A magnetic pole position of the rotor is thereby detected. However, there is friction in an oil seal of the motor and a mechanical system connected to a shaft of the motor. Unless the torque generated by the motor becomes larger than the friction, the rotor will not move. For this reason, in the conventional control system for a synchronous motor, when the magnetic pole position of the rotor becomes close to a proper value and the torque generated by the motor becomes smaller than the friction torque, the rotor of the motor comes to a stop. The correct magnetic pole position of the rotor, where the maximum torque may be obtained, cannot be determined. An error is thereby produced in detection of the magnetic pole position. When there is an error in the detection of magnetic pole position of the rotor (moving element in the case of a linear motor), the phase of a current that flows through armature windings on the stator will be deviated from a phase where the maximum torque may be obtained. Thus, the torque generated by the motor during a regular operation will be reduced. Likewise, since weakened magnetic field compensation is performed based on the magnetic pole position, an amount of the weakened magnetic field compensation differs between forward rotation and reverse rotation of the motor, when there is an error in detection of the magnetic pole position. As a result, the torque generated by the motor may become different between the forward rotation and the reverse rotation of the motor.